HIV-infected current and former drug users have received disproportionately less benefit from antiretroviral therapy than non-drug users, in part because of sub-optimal adherence. Despite the clinical importance of adherence assessments and a large body of recent literature on antiretroviral adherence, there is no single adherence measure that is universally accepted in clinical or research settings. The lack of methodogical rigor in adherence measurement is particularly significant in intervention trials and may lead investigators to draw erroneous conclusions. Composite adherence measures may become the gold standard for assessing the complex and dynamic behavior of medication adherence in research studies, but such measures are only beginning to be evaluated. In clinical settings, imprecise adherence estimates can result in inaccurate targeting of interventions and poor clinical outcomes. Self-report is the most feasible, flexible, and widely used measure, but the relative validity of different self-report measures is unknown. To address these issues, I propose to study adherence measurement in current and former drug users. The specific aims of this proposal are: (1) to develop a composite antiretroviral adherence measurement model that incorporates four methods of measurement (self-report, pill count, pharmacy refill records, and MEMS), and to compare the correlation with HIV viral load of the composite model with the correlation of each single method;(2) to examine the agreement between common antiretroviral adherence self-report measures, and to determine the validity of each measure by assessing its correlation with a new adherence measurement gold standard (composite antiretroviral adherence model) and an adherence surrogate (HIV viral load);and (3) to describe the cognitive processes undertaken by current and former drug users to answer self-report antiretroviral adherence questions, including question interpretation, ability to remember medication taking, and answer editing. My major goal is to become an independent clinical investigator with expertise in behavioral substance abuse research. The career development plan I have proposed includes mentorship in antiretroviral adherence, behavioral research methods, and substance abuse research, coupled with formal coursework and participation in local interactive seminars and national scientific meetings.